1. Field of the Invention
The present invention relates to a conductive elastomer composition, a conductive member such as a conductive roller and a conductive belt formed of the conductive elastomer composition, and an image-forming apparatus having the conductive member mounted therein.
More particularly, the present invention relates to the conductive elastomer composition, which is used effectively for conductive rollers such as a development roller, a charging roller, a transfer roller, and a toner supply roller; and for conductive belts such as a transfer belt. The conductive rollers and the conductive belts are used in the image-forming apparatus such as a copying machine, a printer, a facsimile, and the like. In particular, the present invention provides the conductive elastomer composition, which is ionic-conductive and has a much lower electric resistance than the conventional one.
2. Description of the Related Art
It is necessary to allow conductive members such as the charging roller, the development roller, the toner supply roller, the transfer roller, and the transfer belt used in the image-forming apparatus such as the copying machine, the printer, the facsimile, and the like to have an appropriate stable electric resistance value. As a method of allowing the conductive roller and the conductive belt to be conductive, the following two methods have been used: In one of the methods, an electroconductive elastomer composition containing a conductive filler such as powder of metal oxides or carbon black is used; and in the other method, an ionic-conductive elastomer composition such as urethane rubber, acrylonitrile-butadiene rubber (NBR), and epichlorohydrin rubber is used.
In the case where the electroconductive elastomer composition containing the conductive filler is used, the electric resistance thereof changes rapidly in a semi-conductive region owing to a slight change of an addition amount of the conductive filler. Thus it is very difficult to control the electric resistance thereof. In addition, because it is difficult to disperse the conductive filler uniformly in the elastomer composition, the electric resistance value has variations in the circumferential and longitudinal directions of the conductive roller and inside the conductive belt. Further the electric resistance value of the conductive roller and the conductive belt composed of the electroconductive elastomer composition depends on a voltage applied thereto and is not constant. The electroconductive elastomer composition containing carbon black shows this phenomenon outstandingly.
Thus as the recent tendency, the ionic-conductive elastomer composition is used rather than the electroconductive elastomer composition in the copying machine, the printer and the like required to form a high-image quality by using digital image processing technique and color image processing technique.
However, it is difficult for the conventional art to reduce the electric resistance value by using only the ionic-conductive elastomer. Thus addition of an ionic-conductive filler to the ionic-conductive elastomer component is proposed. For example, addition of metal ions and a quaternary ammonium salt to the ionic-conductive elastomer component are proposed because only the ionic-conductive elastomer composition such as urethane or NBR makes the electric resistance value too high. In addition, other proposals have been made.
For example, in Japanese Patent Application Laid-Open Nos. 2001-214050 and 2001-217009, there are disclosed a crosslinked polymeric solid electrolytic wall made of a conductive polymeric composition consisting of a copolymer containing ethylene oxide at a polymerization ratio not less than 85% and a salt of lithium ions such as a lithium imide salt added to the copolymer.
In Japanese Patent Application Laid-Open No. 2002-226714, there is disclosed a polymeric electricity control rubber composition containing various elastomers and a specified amount of metal salts added thereto with the intermediary of a specific compound.
However, the conductive polymeric composition disclosed in each of Japanese Patent Application Laid-Open Nos. 2001-214050 and 2001-217009 is used for the wall of a battery. Since the polymerization ratio of ethylene oxide is not less than 85%, the conductive polymeric composition has a low water resistance and thus can be used in only a closed system such as the battery. That is, the conductive polymeric composition cannot be used for products such as the conductive roller, the conductive belt, and the like that are used in an open system having normal temperature and humidity. Since the polymerization ratio of the ethylene oxide is so high that the conductive polymeric composition has a high hardness or deterioration in its compression set. Thus it is difficult to use the conductive polymeric composition for general products or the conductive roller and conductive belt.
The polymeric electricity control rubber composition disclosed in Japanese Patent Application Laid-Open No. 2002-226714 contains the specific low-molecular weight compound serving as a medium. Thus it does not have any problems so long as it is used for general-purpose rubber products. However, there is room for improvement in its compression set, stain of a photosensitive member, and the material cost in applying the electric resistance control rubber composition to products such as the conductive roller, the conductive belt, and the like for use in the copying machine, the printer, and the like.
There is a possibility that some kind of additives such as metal ions, quaternary ammonium salts, and the like deteriorate the compression set of the ionic-conductive elastomer composition or stain the photosensitive member and are incapable of reducing the electric resistance value of the ionic-conductive elastomer composition sufficiently. The additives such as the urethane, NBR, and the like have a high electric resistance value. Thus, the use of these additives do not contribute to realization of such a low volume specific resistance value as that specified by the present invention. Therefore these additives are unsuitable for conductive members for use in the copying apparatus, the printer, and the like.
It is conceivable that the ionic-conductive elastomer composition having a lower electric resistance value than the urethane contains ionic-conductive additives such as metal ions and quaternary ammonium salts. But this method does not necessarily provide a sufficiently low electric resistance value because in dependence on the amount and kind of the ionic-conductive additive, there is much possibility that the property of the ionic-conductive elastomer composition such as its compression set may be deteriorated and the ionic-conductive additive causes stain of the photosensitive member. In dependence on the mixing amount and kind of the salt, electric resistance value of the ionic-conductive elastomer composition increases much in successive energization and cannot be put into practical use.
In addition, the conductive members installed on printers of various types are demanded to have various properties besides having a low electric resistance value: They are demanded to have favorable mechanical properties such as sufficient wear resistance and strength. Further when they are used in an electrophotographic system, the conductive members for charging toner and a photosensitive drum are demanded to have appropriate electrostatic property on the surfaces thereof. It is difficult to compose the rubber layer of the conductive member of only one ionic-conductive elastomer. Thus it is necessary to optimize the properties of the rubber layer by adding various elastomers to comply with performance demanded in dependence on use.
However, in the case where the above elastomers is added to the ionic-conductive elastomer obtained by the conventional art to impart the above-described demanded properties to the ionic-conductive elastomer composition, the conductive member composed of the mixture has a high electric resistance value. Therefore the elastomers are mixed with the ionic-conductive elastomer in a range in which the mixture satisfies a predetermined electric resistance value. Consequently the mixture is low in its strength and wear resistance.
The development roller and the charging roller are the central component part of a printer which forms images by utilizing a static electricity. Thus it is necessary to charge toner and the photosensitive member sufficiently. In addition, the development roller and the charging roller are demanded to have a low electric resistance value in terms of the construction of the printer. Therefore a sufficient amount of elastomers cannot be added to the ionic-conductive elastomer, as described above. Thus it is impossible to adjust the electrostatic property of the surface of the development roller, the charging roller, and the like appropriately. Consequently the toner and the photosensitive member cannot be sufficiently charged and hence an appropriate image cannot be obtained. Accordingly image deterioration occurs during the early stages in a long-term use of the printer.